Structural changes of water and water distribution in a sample are difficult to monitor. One manner in which they may be studied is through isotopic substitution. For example, 3H (tritium) may be used to label water and trace it via radioactive decay. However, because it is radioactive, wide application of such a technique is necessarily limited. The other isotope 2H (deuterium) is found in nature as D2O (or heavy water) which was traditionally confined to the domain of nuclear physics and chemistry, for example as a reference in 1H NMR.
One difficulty in conducting D2O-based experiments is that there is no existing direct method of detecting D2O. One can however indirectly see its effect, for example, how differently it reacts with a substrate as compared to water. Thus, methods are needed to detect and monitor D2O both qualitatively and quantitatively.